Resistance welding units are known which include a frame for supporting the laminated magnetic circuit of a transformer which has two primary coils and a secondary circuit which is connected to the electrodes of the welding unit. Generally, in such units, each electrode is connected to the secondary circuit by flexible braids. These braids have the disadvantage of wearing out rapidly as a result of the intensive electrodynamic reactions which they are required to undergo.
In French Pat. No. 2,044,476 issued May 22, 1969, the present inventor discloses a resistance welding unit in which the welding electrodes are electrically connected to a secondary winding of a transformer which has a primary winding for connection to a source of alternating electric current. In this unit, the secondary winding is formed by two parallel bars which are electrically interconnected to one another on one end. The other ends of the bars are connected to the welding electrodes so that in conjunction with the work pieces to be welded, the bars form the secondary of the transformer. The parallel bars are moveable with respect to the primary winding.
The device disclosed in French Pat. No. 2,044,476 provides a welding unit with numerous advantages. First, the secondary circuit makes direct contact with the part to be welded. Also, the secondary circuit is simplified in that it includes only those parts of the two parallel bars which are located outside the magnetic circuit of the transformer, and, in addition, the corresponding portion of the workpiece to be welded. Further, the external secondary circuit has an impedance which remains constant. This device provides a transformer which is of reduced size and requires less operating power. Hence, a saving in electrical energy consumption is enabled, and the cost of each welding operation is reduced.
In present welding techniques and apparatus, the general objective is to provide an apparatus which is as automatic as possible, and to provide weight and size reductions. However, the underlying principle of the welding unit disclosed in French Pat. No. 69 16 670, described above, renders further reductions in size or weight difficult. Furthermore, the secondary circuit of the device disclosed in this patent is formed of two parallel bars which are guided separately and are interconnected in the transformer by a flexible shant. As a result, maintenance operations require the dismantling of the transformer and the provision of two guiding devices for each bar.
In French Pat. No. 2,505,697 filed May 12, 1981, the present inventor provides a resistance welding unit similar to French Pat. No. 2,044,476, but which is more compact, light-weight and requires a minimum of maintenance. This system is directed to a welding unit for direct welding spots. In this system, the transformer consists of a primary winding for connection to an alternating current source and a secondary winding which includes a single turn and which is movable in the transformer. The secondary winding includes a bar having a rectangular U-shaped cross section. One limb of the U-shaped bar is connected by a braid to one electrode and the other limb is rigidly connected to the other electrode. Such device offers significant advantages in that it eliminates the need for a guiding system for the secondary circuit because the secondary circuit is rigidly connected to an electrode which in turn is supported by the jack.
The invention disclosed in French Patent Application No. 2,505,697 relates to a novel welding unit of the "scissors" type, in which the electrodes are pivotally mounted on or articulated to a single common shaft and are connected to a jack, such that the electrodes can be opened and closed with respect to each other. The electrodes are connected to the secondary of the transformer. It can be readily understood that the electrode movements require the provision of flexible braidings. The attendant inconveniences associated with flexible braidings are well known.
U.S. Pat. No. 1,645,705 discloses a welding device which includes a high-power transformer which has secondary outputs which are connected by bars to a shaft for supporting the welding pincers. In this device, one bar is electrically connected via the shaft to one of the electrodes and is insulated from the other electrode, while the second bar is electrically connected via the shaft to the other electrode and is also insulated from the other electrode. Upon initial consideration, this device might appear to offer advantages, but in actuality it is less advantageous than the above-described welding devices for the following reasons. First, the bars result in a substantial lengthening of the secondary circuit, thus increasing electrical impedance. Furthermore, because the bars are in contact with the shaft about which the welding pincers rotate, substantial friction must result; without this substantial friction, the required contact between the bars and the pincers is not provided and the numerous adjustments of the angular position of the pincers are difficult to implement. Finally, in this device, the bars tend to move apart, due to electrodynamic reactions, in the opposite direction to the movement required for satisfactory contact between the parts.
It is, therefore, an object of the present invention to provide an improved electric resistance welding apparatus which substantially overcomes the above-described deficiencies in the prior art devices.